Latinos are the fastest growing ethnic group in the US and are disproportionately impacted by chronic conditions (e.g., overweight/obesity, diabetes) that can be mitigated by physical activity. While evidence-based programs are available to improve physical activity, dissemination is constrained by reliance on costly, limited resources. Short message service (SMS or text messaging) has strong potential as an intervention medium for Latinos, as they are dominant users of mobile phones and text messaging, including those with limited literacy and computer skills. We propose to develop a personalized, culturally-congruent and interactive SMS program to promote the adoption and maintenance of physical activity for overweight, underactive Latino adults and test it for comparable efficacy to an evidence-based telephone-based advising program that has been used successfully with Latinos. Grounded in Social Cognitive and Self-Determination Theories, both programs will include a core set of self-regulatory skill building, social support, and autonomous motivation enhancement, and will be tailored for cultural, linguistic and literacy appropriateness. Randomized at the individual level, 376 sedentary, overweight Latinos ages 45 years and older will be assigned to one of three arms: the SMS- delivered physical activity program, the Human-delivered physical activity program, or an attention-control arm of SMS-delivered basic healthful nutrition advice. They will receive their assigned program for 12 months, and then will be followed for a 6-month maintenance phase, with a final assessment at 18 months. Analyses will determine if the two physical activity programs result in equivalent efficacy in adoption and maintenance of clinically meaningful physical activity increases, and superiority over the attention-control arm at 12 and 18 months. The two physical activity interventions will also be compared for equivalency in prevention of weight gain and reduction in abdominal adiposity. We will explore the relative costs of the programs to determine which program increases physical activity for a lower cost. While health promotion programs delivered by professional staff may be more expensive than those delivered electronically, it is unclear if the automated version can produce more cost-efficient outcomes (i.e., comparable improvements in outcomes for significantly reduced cost). This study will determine if SMS is equivalent in efficacy to the traditional human-delivery channel in increasing physical activity and preventing weight gain, and if it results in a more cost-efficient program. If achieved, this research can significantly reduce health disparities associated with physical activity for Latinos by providing an efficacious tailored and culturally appropriate program poised for widespread dissemination and rapid uptake.